For example, a nonvolatile memory device 500 shown in FIG. 10 is known as a conventional nonvolatile memory device. In the nonvolatile memory device 500, a control gate 54 is provided on a floating gate 51 and an element isolation insulating film 52 via an intergate insulating film 53.
However, according to this nonvolatile memory device 500, the distance (see an arrow 57 in FIG. 10) between an active region 56 of a substrate 55 under the floating gate 51 and the control gate electrode 54 above the element isolation insulating film 52 is reduced by miniaturization. Therefore, a high electric field is applied to this active region during a writing or erasing operation. This may lead to a dielectric breakdown or to damage to the active region.
There is known another nonvolatile memory device in which part of an intergate insulating film in a memory cell region is included in an element isolation trench of a substrate. According to this nonvolatile memory device, part of a control gate electrode is also included in the element isolation trench, so that a large region of the side surface of a floating gate electrode faces the control gate electrode. Thus, this nonvolatile memory device has a high coupling ratio.
However, according to this nonvolatile device, the distance between an active region of the substrate under the floating gate and the control gate electrode above an element isolation insulating film is smaller than that in the nonvolatile memory device 500 described above. Therefore, a dielectric breakdown or damage to the active region may be caused by miniaturization.